Heat Pulse Experiments at Very Low Temperatures in Vitreous Silica

Abstract

According to the most widely accepted mode1 of the amorphous state, which postulates the existence of localized two level tunneling defects, the specific heat at very low temperatures should be time dependent, increasing as the logarithm of the experimental time scale, This important prediction has not yet been unequivocably confirmed, with two negative results (no time dependence) reported2 for time scales of the order of 1–100 microseconds and only our initial work3 on SiO2-Suprasil W at T < 200 mK on a time scale~30mS to 1 S agreeing with the predictions of the generalized tunneling model. However, our more recent work on SiO2 using a different addenda mounting does not agree with the tunneling model nor with the solution of the heat equation assuming constant specific heat (as did the previous negative results). Rather, a time dependent specific heat is implied, but one that decreases with time, in direct contradiction to the tunneling model.